But while I was checking the conventions used for warning and errors in function objects, for fixing report #2441, I stumbled upon a weird situation with at least two files within 'src/functionObjects/field':

After looking up a bit at the OpenFOAM history trees, it seems like there was a previous transition to using an explicit 'UName' convention, but possibly ended up not being the standard convention. But on the aforementioned commit, semi-automated updates were made and resulted in this stale and fairly inconsistent check.

The additional problem here is that it will not properly react when 'U' is undefined, it instead assume it's named 'U', which is inconsistent with the documentation (required: yes).

Given that the description states that 'U' is a required keyword, I will attach in a few minutes a patch (need to check compatibility between dev and 4.x on these two) that simply replaces this outdated check+parse and simply use a single line to do the lookup without a default.
I'm not suggesting a revised backward compatibility check, because I couldn't quickly find any evidence that this was done for OpenFOAM 4.0 to provide these checks, so now it's a bit too late to go adding them all around.